Semiconductor devices FinFET devices with optimized strained-sourece-drain recess profiles and methods of forming the same

ABSTRACT

Semiconductor devices, FinFET devices with optimized strained-source-drain recess profiles and methods of forming the same are provided. One of the semiconductor devices includes a substrate, a gate stack over the substrate and a strained layer in a recess of the substrate and aside the gate stack. Besides, a ratio of a depth at the greatest width of the recess to a width of the gate stack ranges from about 0.5 to 0.7.

BACKGROUND

The semiconductor integrated circuit (IC) industry has experiencedexponential growth. Technological advances in IC materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generation. In the course of ICevolution, functional density (i.e., the number of interconnecteddevices per chip area) has generally increased while geometry size(i.e., the smallest component (or line) that can be created using afabrication process) has decreased. This scaling down process generallyprovides benefits by increasing production efficiency and loweringassociated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,a three dimensional transistor, such as a fin-type field-effecttransistor (FinFET), has been introduced to replace a planar transistor.Although existing FinFET devices and methods of forming FinFET deviceshave been generally adequate for their intended purposes, they have notbeen entirely satisfactory in all respects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the criticaldimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1A to FIG. 1E are schematic cross-sectional views of a method offorming a FinFET device in accordance with some embodiments.

FIG. 2 is a schematic cross-sectional view of a FinFET device inaccordance with alternative embodiments.

FIG. 3 is a flow chart of a method of forming a FinFET device inaccordance with some embodiments.

FIG. 4 is a schematic cross-sectional view of a FinFET device inaccordance with yet alternative embodiments.

FIG. 5 to FIG. 7 are schematic cross-sectional views of semiconductordevices in accordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a second feature over or on a first feature in the description thatfollows may include embodiments in which the second and first featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the second and first features,such that the second and first features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath”, “below”, “lower”,“on”, “over”, “overlying”, “above”, “upper” and the like, may be usedherein for ease of description to describe one element or feature'srelationship to another element(s) or feature(s) as illustrated in thefigures. The spatially relative terms are intended to encompassdifferent orientations of the device in use or operation in addition tothe orientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

FIG. 1A to FIG. 1E are schematic cross-sectional views of a method offorming a FinFET device in accordance with some embodiments.

Referring to FIG. 1A, a substrate 100 with one or more fins 101 isprovided. In some embodiments, the substrate 100 includes asilicon-containing substrate, a silicon-on-insulator (SOI) substrate, ora substrate formed of other suitable semiconductor materials. Dependingon the requirements of design, the substrate 100 may be a P-typesubstrate or an N-type substrate and may have doped regions therein. Thedoped regions may be configured for an N-type FinFET device or a P-typeFinFET device. In some embodiments, the substrate 100 has an isolationlayer formed thereon. Specifically, the isolation layer covers lowerportions of the fins 101 and exposes upper portions of the fins 101. Insome embodiments, the isolation layer is a shallow trench isolation(STI) structure.

In some embodiments, the substrate 100 has at least two gate stacks 107formed thereon. In some embodiments, the gate stacks 107 extend in adirection different from (e.g., perpendicular to) the extendingdirection of the fins 101. In some embodiments, each of the gate stacks107 includes, from bottom to top, an interfacial layer 102, a dummy gate104 and an optional cap pattern 106. In some embodiments, the gatestacks 107 are referred to as “dummy gate stacks” through thedescription. In some embodiments, the interfacial layers 102 includesilicon oxide, silicon oxynitride or a combination thereof, the dummygates 104 include a silicon-containing material, such as polysilicon,amorphous silicon or a combination thereof, and the cap patterns 106include SiN, SiC, SiCN, SiON, SiCON or a combination thereof. In someembodiments, the method of forming the gate stacks 107 includes forminga stacked layer on the substrate 100 and patterning the stacked layerwith photolithography and etching processes. In some embodiments, thegate stacks 107 has a gate width W_(G1) and a spacing width S betweentwo adjacent gate stacks 107.

In some embodiments, the substrate 100 further has spacers 108 formed onsidewalls of the gate stacks 107. In some embodiments, the spacers 108have a dielectric constant less than about 10, or even less than about5. In some embodiments, the spacers 108 include a nitrogen-containingdielectric material, a carbon-containing dielectric material or both. Insome embodiments, the spacers 108 include SiN, SiCN, SiOCN, SiOR(wherein R is an alkyl group such as CH₃, C₂H₅ or C₃H₇), SiC, SiOC,SiON, a combination thereof or the like. In some embodiments, the methodof forming the spacers 108 includes forming a spacer material layer onthe substrate 100, and partially removing the spacer material layer byan anisotropic etching process.

Referring to FIG. 1B, a first etching step is performed to form recesses110 in the substrate 100 between the gate stacks 107. In someembodiments, the first etching step includes performing an anisotropicetching process, such as a dry etching process. In some embodiments, therecesses 110 are formed with a U-shaped profile, a cup-like profile or abowl-like shape. In some embodiments, the top edge of at least one ofthe recesses 110 is aligned with the sidewalls of the adjacent spacers108.

In some embodiments, the ratio of the width W_(G1) of the gate stacks107 to the depth D₀ (measuring from the surface of the substrate 100) ofthe recesses 110 ranges from about 0.4 to 0.7. In some embodiments, theratio of W_(G1) to D₀ can be, for example but is not limited to, about0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, including any range betweenany two of the preceding values.

Referring to FIG. 1C, a second etching step is performed to deepen andwiden the recess 110 and therefore form recesses 111. In someembodiments, the second etching step includes performing an isotropicetching process, such as a dry etching process. In some embodiments,each of the first and second etching steps is referred to as aself-aligned etching process by using the spacers 108 and the cappatterns 106 as self-aligned masks.

In some embodiments, the second etching step deepens the recesses to adepth D₁, and the ratio of the depth D₀ of recesses 110 to the depth D₁of recesses 111 ranges from about 0.7 to 0.9, such as 0.7, 0.75, 0.8,0.85 or 0.9, or any range between any two of the preceding values. Insome embodiments, the second etching step simultaneously enlarges orwidens the middle portion of the recesses to a width W_(M1).Specifically, the recesses 111 are formed with a wide-middle profile ora diamond-like profile after the second etching step. In someembodiments, the middle width W_(M1) of the recesses 111 is greater thaneach of the top width W_(T1) and the bottom width W_(B1) of the recesses111. In some embodiments, the middle width W_(M1) of the recesses 111 isthe greatest width, and the top width W_(T1) is greater than the bottomwidth W_(B1) of the recesses 111, as shown in FIG. 1C.

In some embodiments, the second etching step deepens and widens therecesses without enlarging the top width of the recesses, so the topedge of at least one of the recesses 111 is aligned with the sidewallsof the adjacent spacers 108. In other words, the top edge of each recess111 is not covered by the adjacent spacers 108. However, the disclosureis not limited thereto.

In alternative embodiments, the second etching step not only deepens andwidens the recesses but also enlarges the top width of the recesses, sothe recesses 111 extend to below the spacers 108 after the secondetching step, as shown in FIG. 2. In such case, the top edge of at leastone of the recesses 111 is covered by portions of the adjacent spacers108.

In some embodiments, the etching profile of at least one of the recesses111 defines a tip T substantially positioned at the intersection of theupper sidewall and lower sidewall of the recess 111. In someembodiments, the tip T of each recess 111 is located at a level of thegreatest width of the recess 111. In some embodiments, at least one ofthe recesses 111 further defines a proximity P and a tip depth (orcalled “tip height”) D_(M). The proximity P defines a distance from thetip T to the sidewall of the adjacent gate stack 107. In someembodiments, the proximity P ranges from about zero to 7 nm. In someembodiments, the proximity P is about zero or greater than zero and lessthan or equal to 7 nm for an N-type FinFET device. In alternativeembodiments, the proximity P is about zero or greater than zero and lessthan or equal to 4 nm for a P-type FinFET device. In some embodiments,the greatest width or the middle width W_(M1) of the recesses 111 is nomore than the spacing width S of the gate structures 107. From anotherpoint of view, the tip T of each recess 111 extends to below theadjacent spacers 108, without extending to below the adjacent gatestacks 107.

In some embodiments, the ratio of the depth D_(M) at the greatest widthof the recesses 111 to the total depth D₁ of the recesses 111 rangesfrom about ¼ to ⅓. In some embodiments, the ratio of D_(M) to D₁ can be,for example but is not limited to, about 0.25, 0.27, 0.29, 0.31, 0.33,including any range between any two of the preceding values.

In some embodiments, the ratio of the depth D_(M) at the greatest widthof the recesses 111 to the W_(G1) of the gate stacks 107 ranges fromabout 0.5 to 0.7. In some embodiments, the ratio of D_(M) to W_(G1) canbe, for example but is not limited to, about 0.50, 0.55, 0.60, 0.65,0.70, including any range between any two of the preceding values.

In some embodiments, the ratio of the width W_(G1) of the gate stacks107 to the greatest width or the middle width W_(M1) of the recess 111ranges from about 0.4 to 0.6. In some embodiments, the ratio of W_(G1)to W_(M1) can be, for example but is not limited to, about 0.40, 0.45,0.50, 0.55, 0.60, including any range between any two of the precedingvalues.

In some embodiments, the included angle α formed between the surface ofthe substrate 100 and the upper sidewall of the adjacent recess 111 issubstantially equal to or greater than about 50 degrees and less than 90degrees. In some embodiments, the upper sidewalls of recesses 111 areslightly curved, and the included angle α is the angle between thesurface of the substrate 100 and the tangent line to the upper sidewallof the adjacent recess 111 at the recess opening. For example, theincluded angle α can be, for example but is not limited to, about 50,55, 60, 65, 70, 75, 80, 85, including any range between any two of thepreceding values.

Referring to FIG. 1D, strained layers 112 are formed in the recesses111. In some embodiments, two strained layers 112 are formed beside eachof the gate stacks 107, and one of the strained layers 112 is betweenthe adjacent gate stacks 107. In some embodiments, the strained layers112 include silicon germanium (SiGe) for a P-type FinFET device. Inalternative embodiments, the strained layers 112 include silicon carbon(SiC), silicon phosphate (SiP), SiCP or a SiC/SiP multi-layer structurefor an N-type FinFET device. In some embodiments, the strained layers112 may be optionally implanted with a P-type dopant or an N-type dopantas needed. In some embodiments, the method of forming the strainedlayers 112 includes growing epitaxy layers from the recesses 111.Specifically, the strained layers 112 are formed within the recesses 111and extend upwardly along the sidewalls of the corresponding spacers108. In some embodiments, the tops of the strained layers 112 are abovethe surface of the substrate 100. In alternative embodiments, the topsof the strained layers 112 are substantially coplanar with the surfaceof the substrate 100. In some embodiments, the strained layers 112 canbe referred to as “source/drain regions”.

In some embodiments, the adjacent strained layers 112 at the same sideare separate from each other. In alternative embodiments, the adjacentstrained layers 112 at the same side are connected with each other. Insome embodiments, following the formation of the strained layers 112,silicide layers are formed by siliciding the top portions of thestrained layers 112.

In some embodiments, the strained layers 112 has a shape/profile similarto that of recesses 111. In some embodiments, the strained layers 112has a wide-middle profile or a diamond-like profile. In someembodiments, the top, middle and bottom widths of the strained layers112 are substantially equal to the top, middle and bottom widths of therecesses 111, as shown in FIG. 1D. In alternative embodiments, the top,middle and bottom widths of the strained layers 112 are slightly greaterthan the top, middle and bottom widths of the recesses 111.

It is noted that in some embodiments, the profile of at least one of therecesses 111 is well controlled when the ratio of the gate width (e.g.,W_(G1)) to the initial recess depth (e.g., D₀) and/or the ratio of theinitial recess depth (e.g., D₀) to the final recess depth (e.g., D₁)fall within the above ranges.

It is also noted that in some embodiment, the profile of at least one ofthe recesses 111 is well defined when the ratio of the tip depth (e.g.,D_(M)) of the recess to the gate width (e.g., W_(G1)), the ratio of thegate width (e.g., W_(G1)) to the greatest depth (e.g., W_(M1)) of therecess, the ratio of the tip depth (e.g., D_(M)) to the total depth(e.g., D₁) of the recess, and/or the included angle (e.g., α) betweenthe recess sidewall and the substrate surface fall within the aboveranges. By controlling and adjusting the above ratios and/or theincluded angle within the ranges of the disclosure, the profile of therecesses 111 is well defined, the subsequently formed strained layers112 are suitable to impart more stress on the channel regions, and theelectrical performance of the device is accordingly boosted.

Specifically, when the ratios of D_(M) to W_(G1), W_(G1) to W_(M1) andD_(M) to D₁ and the included angle α are over the upper limits, thesaturation current is too low. When the ratios of D_(M) to W_(G1),W_(G1) to W_(M1) and D_(M) to D₁ and the included angle α are below thelower limits, the leakage current may easily occur because the dummygates may be damaged during the recess forming step.

Referring to FIG. 1E, a dielectric layer 113 is formed aside the gatestacks 107 and over the strained layers 112. In some embodiments, thedielectric layer 113 includes nitride such as silicon nitride, oxidesuch as silicon oxide, phosphosilicate glass (PSG), borosilicate glass(BSG), boron-doped phosphosilicate glass (BPSG), a combination thereofor the like. In some embodiments, the top surface of the dielectriclayer 113 is substantially level with the top surfaces of the gatestacks 107. The dielectric layer 113 may be filled until its top surfaceis higher than the top surfaces of the cap patterns 106 by a suitablefabrication technique such as spin-coating, CVD, flowable CVD, PECVD,ALD, a combination or the like. A planarization step such as CMP is thenperformed to remove the excess dielectric layer. In some embodiments, acontact etch stop layer (CESL) is formed after the step of forming thestrained layers 112 and before the step of forming the dielectric layer113, and the CESL includes SiN, SiC or the like.

Thereafter, the dummy gate stacks 107 are replaced with gate stacks 117.In some embodiments, the gate stacks 117 are referred to as “metal gatestacks” through the description. In some embodiments, the gate stacks107 are removed to form gate trenches in the dielectric layer 113, andthe gate stacks 117 are then formed in the gate trenches. In someembodiments, the method of forming the gate stacks 117 includes forminga stacked layer with CVD, PVD, plating, or a suitable process, and thenperforming a CMP process to remove the stacked layer outside of the gatetrenches. In some embodiments, the top surface of the dielectric layer113 is substantially level with the top surfaces of the gate stacks 117.In some embodiments, the shape, profile and width of the gate stacks 117are substantially similar to the shape, profile and width of the gatestacks 107.

In some embodiments, each of the gate stacks 117 includes a gatedielectric layer 114 and a gate 116 (or called “replacement gate”). Insome embodiments, the gate stacks 117 extend in a direction differentfrom (e.g., perpendicular to) the extending direction of the fins 101.In some embodiments, each of the gate dielectric layers 114 surroundsthe sidewall and bottom of the corresponding gate 116 and on the top andsidewall of each fin 101, as shown in FIG. 1E. In some embodiments,silicon oxide layers are formed between the fins 101 and the gatedielectric layers 114.

In some embodiments, each of the gate dielectric layers 114 includes ahigh-k material having a dielectric constant greater than about 10. Insome embodiments, the high-k material includes metal oxide, such asZrO₂, Gd₂O₃, HfO₂, BaTiO₃, Al₂O₃, LaO₂, TiO₂, Ta₂O₅, Y₂O₃, STO, BTO,BaZrO, HfZrO, HfLaO, HfTaO, HfTiO, a combination thereof, or a suitablematerial. In alternative embodiments, each of the gate dielectric layers114 can optionally include a silicate such as HfSiO, LaSiO, AlSiO, acombination thereof, or a suitable material.

In some embodiments, each of the gates 116 includes a metal materialsuitable for forming a metal gate or portion thereof. In someembodiments, each of the gates 116 includes a work function metal layerand a fill metal layer on the work function metal layer. The workfunction metal layer is an N-type work function metal layer or a P-typework function metal layer. In some embodiments, the N-type work functionmetal layer includes TiAl, TiAlN, or TaCN, conductive metal oxide,and/or a suitable material. In alternative embodiments, the P-type workfunction metal layer includes TiN, WN, TaN, conductive metal oxide,and/or a suitable material. The fill metal layer includes copper,aluminum, tungsten, or a suitable material. In some embodiments, each ofthe gates 116 can further include a liner layer, an interface layer, aseed layer, an adhesion layer, a barrier layer, a combination thereof orthe like. The FinFET device 10 of the disclosure is thus completed.

The above-mentioned process steps in FIG. 1A to FIG. 1E can be conciselyillustrated with reference to the flow chart of FIG. 3.

At step 200, a substrate 100 is provided with a gate stack 107 formedthereon, as shown in FIG. 1A. At step 202, a first etching step isperformed to form a recess 110 in the substrate 100 aside the gate stack107, as shown in FIG. 1B. In some embodiments, the first etching stepincludes performing an anisotropic etching process. In some embodiments,the ratio of the width W_(G1) of the gate stack 107 to the depth D₀ ofthe recess 110 ranges from about 0.4 to 0.7. At step 204, a secondetching step is performed to deepen and widen the recess 110 to form arecess 111, as shown in FIG. 1C. In some embodiments, the second etchingstep includes performing an isotropic etching process. At step 206, astrained layer 112 is formed in the recess 111, as shown in FIG. 1D. Atstep 208, the gate stack 107 is replaced with a metal gate stack 117, asshown in FIG. 1E. The FinFET device of the disclosure is thus completedwith the described process steps. However, it is not limited to add oneor more additional steps among the above steps for fabricating theFinFET device.

In the above embodiments, the recesses 111 (or strained layers 112) areformed with a wide-middle profile in which the top width is smaller thanthe middle width of the recesses 111 (or strained layers 112), and thus,the included angle α between the upper sidewall of each recess 111 (orstrained layer 112) and the surface of the substrate 100 is in a rangeof substantially equal to or greater than 50 degrees and less than 90degrees. However, the present disclosure is not limited thereto. Inalternative embodiments, as shown in FIG. 4, the recesses 111 (orstrained layers 112) are formed with a bucket-like profile in which thetop width is substantially equal to the middle width of the recesses 111(or strained layers 112), and thus, the included angle α between theupper sidewall of each recess 111 (or strained layer 112) and thesurface of the substrate 100 is about 90 degrees.

The structures of the FinFET devices of the disclosure are describedbelow with reference to FIG. 1E, FIG. 2 and FIG. 4. In some embodiments,the FinFET device 10/20/30 of the disclosure includes a substrate 100having at least one fin 101, two gate stacks 117 across the at least onefin 101, and a strained layer 112 in a recess 111 of the substrate 100and between the gate stacks 117.

In some embodiments, the ratio of a depth D_(M) at the greatest width ofthe recess 111 to a width W_(G1) of the gate stack 117 ranges from about0.5 to 0.7. In some embodiments, the ratio of a width W_(G1) of the gatestacks 117 to the greatest width W_(M1) of the recess 111 ranges fromabout 0.4 to 0.6. In some embodiments, the ratio of a depth D_(M) at thegreatest width of the recess 111 to a total depth D₁ of the recess 111ranges from about ¼ to ⅓.

In some embodiments, the strained layers 112 have a wide-middle profileor a bucket-like profile. In some embodiments, the included angle αbetween an upper sidewall of the recess 111 and a surface of thesubstrate 100 is from about 50 degrees to 90 degrees. In someembodiments, the top width is smaller than the middle width of therecess 111, as shown in FIG. 1E and FIG. 2. In alternative embodiments,the top width is substantially equal to the middle width of the recess111, as shown in FIG. 4.

In some embodiments, the FinFET device 10/20/30 of the disclosurefurther includes spacers 108 on sidewalls of the gate stacks 117. Insome embodiments, a top edge of the recess 111 is aligned with thesidewalls of the spacers 108, as shown in FIG. 1E. In alternativeembodiments, a top edge of the recess 111 is covered by the spacers 108,as shown in FIG. 2 and FIG. 4. Besides, the greatest width W_(M1) of therecess 111 is no more than a distance (e.g., spacing width S) betweenthe gate stacks 117.

The above embodiments in which each of the gate dielectric layers, thegates, the spacers, the strained layers and the dielectric layer is asingle layer are provided for illustration purposes, and are notconstrued as limiting the present disclosure. In some embodiments, atleast one of these described elements can be a multi-layer structure asneeded.

In the above embodiments, the method of the disclosure is applied to aFinFET device process, but the disclosure in not limited thereto. Inalternative embodiments, the method of disclosure can be applied to aplanar device process.

FIG. 5 to FIG. 7 are cross-sectional views of semiconductor devices inaccordance with alternative embodiments.

As shown in FIG. 5 to FIG. 7, a semiconductor device 40/50/60 includes aplanar substrate 300, gate stacks 305, spacers 308 and strained layers312. In some embodiments, each of the gate stacks 305 includes a gatedielectric layer 302 on the planar substrate 300 and a gate 304 on thegate dielectric layer 302. In some embodiments, the gate dielectriclayer 302 includes silicon oxide, a high-k material or a combinationthereof. The gate 304 includes a silicon-containing material, ametal-containing material or a combination thereof. The spacers 308 areformed on sidewalls of the gate stacks 305.

The gates stacks 305, spacers 308 and strained layers 312 are similar tothe gate stacks 107, spacers 108 and strained layers 112, and thedetails are not iterated herein. Specifically, the structures of FIG. 5,FIG. 6 and FIG. 7 are similar to the structures of FIG. 1E, FIG. 2 andFIG. 4, and the difference between them lies in that the formerstructures are formed on a planar substrate without fins, while thelater structures are formed on a substrate with fins. Therefore, theratios, shapes and relationships between elements of FIG. 1E, FIG. 2 andFIG. 4 are applicable to the structures of FIG. 5, FIG. 6 and FIG. 7.

In the above-mentioned embodiments, a “gate last” process is implementedto form a FinFET device. However, another process such as a “gate first”process can be applied by using similar processes as described herein.The methods disclosed herein can be easily integrated with a CMOSprocess flow and do not require additional complicated steps to achievethe desired results. It is understood that embodiments disclosed hereinoffer different advantages, and that no particular advantage isnecessarily required for all embodiments.

In view of the above, in some embodiments, by controlling and adjustingthe ratio of the tip depth (e.g., D_(M)) of the recess to the gate width(e.g., W_(G1)), the ratio of the gate width (e.g., W_(G1)) to thegreatest depth (e.g., W_(M1)) of the recess, the ratio of the tip depth(e.g., D_(M)) to the total depth (e.g., D₁) of the recess, and/or theincluded angle (e.g., a) between the recess sidewall and the substratesurface within the specific ranges of the disclosure, the profiles ofthe recesses and therefore the strained layers are well defined.Therefore, the strained layers are suitable to impart more stress on thechannel region, and the electrical performance of the device isaccordingly boosted.

In accordance with some embodiments of the present disclosure, a FinFETdevice includes a substrate, a gate stack over the substrate and astrained layer in a recess of the substrate and aside the gate stack.Besides, a ratio of a depth at the greatest width of the recess to awidth of the gate stack ranges from about 0.5 to 0.7.

In accordance with alternative embodiments of the present disclosure, aFinFET device includes a substrate having at least one fin, two gatestacks across the at least one fin, and a strained layer in a recess ofthe substrate and between the gate stacks. Besides, a ratio of a widthof the gate stacks to the greatest width of the recess ranges from about0.4 to 0.6.

In accordance with yet alternative embodiments of the present disclosurea method of forming a FinFET device including the following steps. Asubstrate having a gate stack formed thereon is provided. A firstetching step is performed to form a recess in the substrate aside thegate stack, wherein a ratio of a width of the gate stack to a depth ofthe recess ranges from about 0.4 to 0.7. A second etching step isperformed to deepen and widen the recess. A strained layer is formed inthe recess.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a substrate;a gate stack over the substrate; and a strained layer in a recess of thesubstrate and aside the gate stack, wherein a ratio of a depth at thegreatest width of the recess to a width of the gate stack ranges fromabout 0.5 to 0.7, wherein the semiconductor device further comprises aspacer on a sidewall of the gate stack, a top edge of the recess iscovered by and spaced apart from an outer edge of the spacer, a sidewallof the recess connecting the top edge and a bottom edge of the recess isa curved surface without a sharp turning point, and an included anglebetween an upper portion of the sidewall of the recess and a surface ofthe substrate is from about 50 degrees to 90 degrees, and wherein thesubstrate is a substrate with at least one fin extending in a firstdirection, and the gate stack extends in a second direction differentfrom the first direction and is across the at least one fin.
 2. Thesemiconductor device of claim 1, wherein a top width of the recess issmaller than a middle width of the recess.
 3. A FinFET device,comprising: a substrate having at least one fin; two gate stacks acrossthe at least one fin; and a strained layer in a recess of the substrateand between the gate stacks, wherein a ratio of a width of the gatestacks to the greatest width of the recess ranges from about 0.4 to 0.6,wherein the FinFET device further comprises spacers on sidewalls of thegate stacks, a top edge of the recess is covered by and spaced apartfrom outer edges of the spacers, a sidewall of the recess connecting thetop edge and a bottom edge of the recess is a curved surface without asharp turning point, and an included angle between an upper portion ofthe sidewall of the recess and a surface of the substrate is from about50 degrees to 90 degrees.
 4. The FinFET device of claim 3, wherein aratio of a depth at the greatest width of the recess to the width of thegate stacks ranges from about 0.5 to 0.7.
 5. The FinFET device of claim3, wherein a ratio of a depth at the greatest width of the recess to atotal depth of the recess ranges from about ¼ to ⅓.
 6. The FinFET deviceof claim 3, wherein a middle width of the recess is greater than each ofa top width and a bottom width of the recess.
 7. The FinFET device ofclaim 3, wherein the greatest width of the recess is no more than adistance between the gate stacks.
 8. A method of forming a FinFETdevice, comprising: providing a substrate having a gate stack formedthereon; performing a first etching step to form a recess in thesubstrate aside the gate stack, wherein a ratio of a width of the gatestack to a depth of the recess ranges from about 0.4 to 0.7; performinga second etching step to deepen and widen the recess, wherein a top edgeof the recess after the second etching step is covered by and spacedapart from an outer edge of a spacer on a sidewall of the gate stack, asidewall of the recess connecting the top edge and a bottom edge of therecess is a curved surface without a sharp turning point, and anincluded angle between an upper portion of the sidewall of the recessand a surface of the substrate is from about 50 degrees to 90 degrees;and forming a strained layer in the recess.
 9. The method of claim 8,wherein the first etching step comprises an anisotropic etching process.10. The method of claim 8, wherein the second etching step comprises anisotropic etching process.
 11. The method of claim 8, wherein the recesshas a wide-middle profile after the second etching step.